The invention resides in an analysis system comprising at least two light sources with a beam splitter for combining the beams of the light sources, an optical cell and a photodiode array spectrometer.
In a spectral photometer, the spectrum emitted from light sources such as incandescent lamps over a certain wave length range as well as the spectral sensitivity of the sensors used for the detection cause excessive signal differences within the spectral range to be covered. An extreme case resides in the use of incandescent lamps together with silicon photodiode arrays: It is well known that the intensity of an incandescent lamp drops strongly toward blue. At the same time, the sensitivity of silicon photodiodes decreases toward the blue. As a result, the dynamic range of the photodiode arrays used as multi-channel detector is not well utilized which results in a noise with spectral dependency. But even more disturbing is the error light at the short wave end of the measuring range since, at this end, the intensity is very low and barely higher than that of the stray light. In the given example, the stray light is particularly pronounced since an incandescent lamp has a relatively high emission in the close infrared range and silicon photo diodes have their greatest sensitivity in the same area. If such a spectrometer system is used for absorption measurements, the absorption spectrum of a gas, a liquid or a solid to be investigated is highly adulterated so that a quantitative concentration determination using the short wave measurement range is not possible.
A solution frequently used to solve this dilemma resides in employing a halogen lamp. Because of the halogen cycling process in effect in halogen lamps, these lamps can be operated at very high filament temperatures which results not only in a high general light output, but also in a relatively high output of light in the blue spectral range. Unfortunately, the life of halogen lamps is limited to about 1500 hours (2 months), which results in relatively high maintenance costs for a process measuring apparatus which is in constant operation, day and night. Also, the life of a halogen lamp can be extended only insignificantly by operating the lamp at lower than design voltage since the voltage operating range is very limited: at much reduced voltage, the filament and glass bulb temperatures become so low that the halogen cycle process does not operate properly and deposits (blackening) form at the inside of the glass bulb.
To compensate for the signal loss in the short wave range of the visible light the following methods are known:
In scanning spectral photometers which include a detector element and a rotating diffraction screen, a second beam path with a second detector is utilized. Depending on the wave length and consequently the signal intensity, the amplifier for the first detector, with photomultipliers, the high voltage is adjusted. In addition, a deuterium lamp is utilized which provides for light of 200 nm to 400 nm wave length.
This solution however has several disadvantages: for one, this concept may be suitable for laboratory equipment but, because of the mechanical structure (rotating screen), not for rough industrial applications. Also disadvantageous is the short incandescent lamp life, since the lamp has to be operated at a relatively high load in order to generate sufficient blue light radiation.
Also, a deuterium lamp has only a relatively short life of about 3 months when operated constantly. In addition, a scanning system, also in the laboratory, has never the good wave length reproducibility which a rigid solid array spectrometer system has. Finally, the deuterium lamp which is needed for such arrangements is extremely expensive.
Array spectrometers with rigid diffraction screen and a multi-channel detector (photodiode array) are more sturdy and have a good wave length reproducibility. The varying signal intensity is at least partially compensated for by the combination of halogen and deuterium lamps.
The main disadvantages are the short lives of both lamps and the costs of the deuterium lamp.
More advantageous are UV array spectrometers which operate with a single light source, that is, a xenon flash light as described in DE Patent 4 232 371 C2.
The only disadvantage of this arrangement resides in the costs for the xenon flashlight and the necessary transfer optical system compensating for the movements of the light-generating arc. These expenses would be justifiable for the UV light range, but not for the visible light range.
It is also possible to arrange a diaphragm directly in front of the photodiode array which includes restrictions to compensate for array elements which have high light intensities or for photodiodes which are very sensitive. A corresponding procedure is described in EP 0 260 013 A3. However, such arrangements requires expensive adjustment devices and generate additional stray light.
From Yair Talmi, publication Applied Optics, volume 19, issue 9, May 1, 1980, pages 1401 ff, it is further known to improve the dynamic range of array spectrometers by periodically changing the integration time of the array. For example the strong signal range can be evaluated with a short integration duration, whereas the weaker signal ranges can be evaluated with longer light collection periods. The time spectrum is obtained with this method by combining the not over-controlled spectral ranges of all the records of different radiation durations.
This procedure, however, has the disadvantage that, on one hand, a complicated electronic system for controlling the arrays is needed and, on the other hand, the dark current changes differently with regard to the spectrum, and the signal-to-noise ratio deteriorates.
It is the object of the present invention to provide an analysis system with an optical light source which, together with an array spectrometer system better utilize the dynamic range of the photodiode array in a simple manner and, as a result, reduces stray light.